package org.apache.lucene.search;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.io.Serializable;

import org.apache.lucene.index.IndexReader;

/**
 * Expert: Calculate query weights and build query scorers.
 * <p>
 * The purpose of {@link Weight} is to ensure searching does not
 * modify a {@link Query}, so that a {@link Query} instance can be reused. <br>
 * {@link Searcher} dependent state of the query should reside in the
 * {@link Weight}. <br>
 * {@link IndexReader} dependent state should reside in the {@link Scorer}.
 * <p>
 * A <code>Weight</code> is used in the following way:
 * <ol>
 * <li>A <code>Weight</code> is constructed by a top-level query, given a
 * <code>Searcher</code> ({@link Query#createWeight(Searcher)}).
 * <li>The {@link #sumOfSquaredWeights()} method is called on the
 * <code>Weight</code> to compute the query normalization factor
 * {@link Similarity#queryNorm(float)} of the query clauses contained in the
 * query.
 * <li>The query normalization factor is passed to {@link #normalize(float)}. At
 * this point the weighting is complete.
 * <li>A <code>Scorer</code> is constructed by {@link #scorer(IndexReader,boolean,boolean)}.
 * </ol>
 * 
 * @since 2.9
 */
public abstract class Weight implements Serializable {

    /**
     * An explanation of the score computation for the named document.
     * 
     * @param reader sub-reader containing the give doc
     * @param doc
     * @return an Explanation for the score
     * @throws IOException
     */
    public abstract Explanation explain(IndexReader reader, int doc) throws IOException;

    /** The query that this concerns. */
    public abstract Query getQuery();

    /** The weight for this query. */
    public abstract float getValue();

    /** Assigns the query normalization factor to this. */
    public abstract void normalize(float norm);

    /**
     * Returns a {@link Scorer} which scores documents in/out-of order according
     * to <code>scoreDocsInOrder</code>.
     * <p>
     * <b>NOTE:</b> even if <code>scoreDocsInOrder</code> is false, it is
     * recommended to check whether the returned <code>Scorer</code> indeed scores
     * documents out of order (i.e., call {@link #scoresDocsOutOfOrder()}), as
     * some <code>Scorer</code> implementations will always return documents
     * in-order.<br>
     * <b>NOTE:</b> null can be returned if no documents will be scored by this
     * query.
     * 
     * @param reader
     *          the {@link IndexReader} for which to return the {@link Scorer}.
     * @param scoreDocsInOrder
     *          specifies whether in-order scoring of documents is required. Note
     *          that if set to false (i.e., out-of-order scoring is required),
     *          this method can return whatever scoring mode it supports, as every
     *          in-order scorer is also an out-of-order one. However, an
     *          out-of-order scorer may not support {@link Scorer#nextDoc()}
     *          and/or {@link Scorer#advance(int)}, therefore it is recommended to
     *          request an in-order scorer if use of these methods is required.
     * @param topScorer
     *          if true, {@link Scorer#score(Collector)} will be called; if false,
     *          {@link Scorer#nextDoc()} and/or {@link Scorer#advance(int)} will
     *          be called.
     * @return a {@link Scorer} which scores documents in/out-of order.
     * @throws IOException
     */
    public abstract Scorer scorer(IndexReader reader, boolean scoreDocsInOrder, boolean topScorer) throws IOException;

    /** The sum of squared weights of contained query clauses. */
    public abstract float sumOfSquaredWeights() throws IOException;

    /**
     * Returns true iff this implementation scores docs only out of order. This
     * method is used in conjunction with {@link Collector}'s
     * {@link Collector#acceptsDocsOutOfOrder() acceptsDocsOutOfOrder} and
     * {@link #scorer(org.apache.lucene.index.IndexReader, boolean, boolean)} to
     * create a matching {@link Scorer} instance for a given {@link Collector}, or
     * vice versa.
     * <p>
     * <b>NOTE:</b> the default implementation returns <code>false</code>, i.e.
     * the <code>Scorer</code> scores documents in-order.
     */
    public boolean scoresDocsOutOfOrder() {
        return false;
    }

}
